LibJS: Add JSON.stringify

Author: https://github.com/mattco98
Commit: 39576b2238
Pull-request: https://github.com/SerenityOS/serenity/pull/2545
Reviewed-by: https://github.com/Dexesttp
Reviewed-by: https://github.com/awesomekling

Libraries/LibJS/Runtime/ProxyObject.h

@@ -58,6 +58,7 @@ private:
     virtual void visit_children(Visitor&) override;
     virtual const char* class_name() const override { return "ProxyObject"; }
     virtual bool is_proxy_object() const override { return true; }
+    virtual bool is_array() const override { return m_target.is_array(); };
 
     Object& m_target;
     Object& m_handler;